The present invention relates to stabilizer compositions for vinyl halide resin compositions and, more particularly, to stabilizer compositions comprising organotinthioalkanol compounds that are stabilized against gelation and precipitation.
Vinyl halide resins and compositions containing vinyl halide resins are converted to useful articles by heating to soften or melt the resin, and then forming the desired product by thermoforming, i.e. converting the powdery resin composition to a molten mass which is then exposed to pressure to achieve the desired shape. The commercial processes for converting vinyl halide resins include calendering, in which the heated molten mass is converted to sheet or film utilizing a series of pressure generating rollers and take off rolls; extrusion, in which one or more screws convey the molten mass through a cylindrical barrel from which it is forced through shaping dies to form pipes, rods, sheet, profiles or tubes; extrusion blow molding, in which the molten mass is first extruded as a hollow tube called a parision which is then clamped in a mold chamber where compressed air is injected into the sealed tube to blow the item into the shape of the mold to form bottles, toys and other hollow items; and injection molding, in which the molten mass is pushed under high pressure into a mold cavity where it is subsequently chilled to form a solid article such as pipe fittings, electrical boxes, and other shapes.
Vinyl halide resin compositions are also used to coat paper and cloth, which may or may not be embossed. Additional processes for the conversion of vinyl halide resin compositions to useful articles include slush molding and fiber extrusion.
All of the processes for the conversion of vinyl halide resin compositions into useful articles require the use of heat to soften the vinyl halide resin so that it may be formed into the desired articles The heat may be generated by friction developed in the compound, as in extrusion, or externally applied heat such as the heated rolls of a calender. The heat required to soften the vinyl halide resin compositions varies with the compositions but is typically 165.degree. C. to 210.degree. C. Highly plasticized compounds such as calendered flexible sheets would use the lower temperature, while rigid or unplasticized vinyl halide resin compositions generally require the higher range of heat. Pipe, house siding, window profiles and rigid extruded sheet are normally formed at temperatures of 195.degree. to 210.degree. C.
It is well known that vinyl halide resins degrade in the presence of heat greater that 100.degree. C. In fact some degradation occurs at even lower temperatures. Degradation of vinyl halide resins results in undesirable color changes and loss of physical properties. In extreme cases the vinyl halide resin compositions will turn black and become so brittle as to be unusable.
In order to protect the vinyl halide resin compositions against heat and light degradation, stabilizer materials are added. Compounds which have been used to stabilize vinyl halide resin compositions against heat degradation constitutes a broad range of materials and are well documented in the literature, e.g., the Encyclopedia of PVC 2nd Edition, 1986, Nass & Heiberger. Typical stabilizers include lead salts and soaps, barium and cadmium soaps and phenolates, zinc soaps, antimony mercaptides, polyols, pentaerythritol, phosphites, epoxy compounds, amino compounds, magnesium compounds, calcium compounds and organotins.
As described in U.S. Pat. No. 3,919,168, it is well known in the art that organotin sulfur containing compounds, such as the organotin mercaptides, are among the most efficient (by weight) heat stabilizers for vinyl halide resins. U.S. Pat. No. 3,919,168 describes sulfur containing organotins, all of which are characterized by having at least one organic group bonded directly to tin through carbon and at least one sulfur atom bonded alone to tin or a residue of a sulfur containing group such as a mercaptan, or a mercaptoester bonded to tin through sulfur. Both sulfur alone bonded to the organotin and the residue of a sulfur containing moiety may be bonded to the same organotin moiety.
Liquid organotin compounds containing thioalkanol groups bonded to tin through sulfur are also known to be effective stabilizers for vinyl halide resin compositions. U.S. Pat. Nos. 4,059,562 and 4,254,017 disclose organotin compounds which contain thioalkanol groups bonded to tin through sulfur, and disclose their use as heat stabilizing compounds for vinyl halide resins. Organotin ethanolmercaptides represent one class of organotinthioalkanols.
While organotinthioalkanols are highly effective stabilizers for vinyl halide resin compositions, they have several serious shortcomings which have prevented widespread production and commercialization. The most serious deficiency of this class of compounds and compositions is their poor storage stability. Upon standing, in most instances even for a few days, the organotinthioalkanols form viscous, gelatinous mixtures and frequently precipitation occurs. The resultant nonhomogenous compositions are difficult to handle, i.e., to pump and meter and, as a result, it is not possible to achieve the uniformity required from batch to batch in a vinyl resin composition. Further the solid precipitate must be filtered out and discarded as waste, which results in a loss of 10% to 20% of this costly product.
An additional serious deficiency of organotinthioalkanols is evident in manufacture. The viscous materials are very difficult to stir and mix and precipitates block transfer lines and contaminate storage vessels. Extra steps must be taken after each production batch to clean out reactors, transfer lines, filters and storage vessels.
As described in U.S. Pat. No. 4,059,562, Hoch et al. propose to solve the viscosity and precipitation problems of organotinthioalkanols by dissolving the organotin in 10% to 60% by weight (of the organotin) of a liquid alcohol component comprising a glycol having 2 to 10 carbon atoms and 0.1 to 1% by weight (of the organotin) of an alkyl acid phosphate. See col. 4, lines 48-62.
Hoch et al. at col. 3, lines 66- 68 and col. 4, lines 1-3, disclose that particularly good results have been obtained when the liquid alcohol component of the stabilizer system contained from 50 to 100% by weight of hexylene glycol and up to 50% by weight of one or more straight chain and or branched chain monohydric alcohols having 8 to 15 carbon atoms.
Employing relatively large amounts of water miscible solvents, 10% to 60% based on the weight of the organotinthioalkanol, as proposed by Hoch et al., is undesirable in the majority of applications for vinyl halide resin stabilization since water resistance of the vinyl halide resin compositions is decreased in the presence of such materials.
It is known in the art that precipitation in organotin mercaptoacid esters can be eliminated by employing a stoichiometric excess of the organotin chloride during the reaction with the mercaptoacid ester. It is also known, as described in U.S. Pat. No. 3,715,333 to Larkin, to use mixtures of organotin chlorides and organotin mercaptoacetates or mercaptides as stabilizers for vinyl halide resins. However, organotin chlorides alone do not eliminate the precipitation problem encountered with conventional organotinthioalkanol stabilizers.
Thus, there is a need for shelf stable organotinthioalkanol stabilizer compositions that are stabilized against gelation and precipitation.